In recent years, active matrix display devices (such as liquid crystal display devices, light-emitting display devices, or electrophoretic display devices) in which a switching element including a TFT is provided in each of display pixels arranged in a matrix have been actively developed. In the active matrix display devices, a switching element is provided in each of pixels (or each of dots), and thus, there is such an advantage that the active matrix display devices can be driven at lower voltage than passive matrix display devices in the case where the pixel density is increased.
In addition, a technique has attracted attention, where a thin film transistor (TFT) in which a channel formation region is formed using an oxide semiconductor film, or the like is manufactured and such a TFT or the like is applied to electronic devices or optical devices. For example, a TFT in which zinc oxide (ZnO) is used as an oxide semiconductor film or a TFT in which InGaO3(ZnO)m is used as an oxide semiconductor film can be given. A technique in which a TFT including such an oxide semiconductor film is formed over a light-transmitting substrate and used as a switching element or the like of an image display device, is disclosed in Reference 1 and Reference 2.